Weight percent is the weight of solute divided by the weight of the solution. In this case, the solute is salt, the solvent is water and the solution is salt solution.

First you find the mass of the salt inside the solution. You have the weight percentage as well as the total weight, so you can easily find the mass of the salt.

Next, you set up the fraction expression to find the original weight percent. This means,
(mass of salt)/(mass of salt + mass of water) = 0.4

When you add the additional salt, you add on both the top and the bottom of the fractions, as adding the salt will also change the mass of the solution. In this case, I made X become the additional mass of salt. With this, the fraction now equals to 0.65.

From there, you should be able to solve for X, hence the mass of the additional salt. Give it a try. Hope it helps